1. Field of the Invention
The present invention relates to a data switching system for a satellite telecommunication system including a plurality of user terminals sending data to a plurality of coverage areas. The data can be transmitted in the form of packets and switched by the switching system on board a satellite. The satellite can be a geostationary or non-geostationary satellite. The packets can be asynchronous transfer mode (ATM) cells, but the device can be adapted for any type of fixed length or variable length packet.
The invention also relates to a transmission device, a transmission method, and a switching method.
2. Description of the Prior Art
A telecommunication system 100 shown in FIG. 1 includes a plurality of user terminals 2, 7 that take the form of ground stations communicating with each other via a satellite 3 with a switch 11 on board. The role of the satellite 3 is to provide very long links 6 where the investment in cable would be unrealistic for financial or technical reasons. The onboard switch 11 therefore receives at its input ports uplinked data or data, i.e. data uplinked from the various ground stations 2 to the satellite 3, and distributes from its output ports downlinked data, i.e. data downlinked from the satellite 3 to other ground stations 7. The terminals 2 that send to the same input port of the switch 11 are grouped in the same geographical coverage area 1, also referred to as a spot or beam. Similarly, the terminals 7 that receive data from the same output port of the switch 11 are grouped in a coverage area 8. The coverage areas are not necessarily separate: it is possible for a terminal 7 to be in several coverage areas at the same time, for example. In particular, the coverage area can transport a stream of data whose final destination is common to a plurality of terminals. The switch 11 advantageously switches the data stream to the output port connected to the common coverage area instead of duplicating said data stream to the various coverage areas, thereby economizing on downlink resources. This facility can be used for multicast data streams, for example, or for collective control data.
The user terminals 2 are very often in competition for resources, i.e. for the uplink and downlink bandwidth of the satellite 3.
Many devices known in the art offer a solution to the problem of dynamic management of the uplink and downlink resources of a satellite system providing dynamic connectivity between coverage areas via the satellite.
One solution to uplink resource management is to use a demand assignment multiple access (DAMA) controller based on a dynamic resource allocation protocol which assigns user terminals frequencies and time slots when said terminals express the requirement to send data in the form of packets on uplinks from a terminal to a satellite by sending requests to that effect to the DAMA controller. A switch on board the satellite then distributes data packets arriving on a plurality of uplinks to a plurality of downlinks.
In the case of downlink resource management, a distinction is drawn between two categories of satellite systems providing dynamic connectivity between coverage areas via the satellite. A first solution consists of making the uplink access patterns and the downlink access patterns completely compatible for a given period during which the switch effects deterministic and a priori switching of the data streams at each of its outputs. There is no situation of conflict for access to the downlink resources since the controller defines the patterns to achieve this. The calculation of the compatibility of the uplink and downlink access patterns results from a synthesis of all the requests from user terminals with the available resources. This calculation is effected by a controller, which can be an onboard controller, although this is not essential, for a defined period during which the patterns are fixed. Any modification of the characteristics of the uplink data stream from a user terminal (for example the bit rate or the destination) generates a new request from said terminal to the switching controller. The controller then proposes new access patterns to the uplinks and the downlinks, compatible with the new configuration. As a result of this, the user terminals are highly interdependent. One consequence is to impede the agility with which the satellite system can respond to a modification of the characteristics of the data stream. This solution uses “deterministic” switching: the position of the packet of the data in an uplink frame pattern determines its destination, so that no address analysis is necessary on board the satellite. This reduces the complexity on the satellite's onboard systems, and the switching unit can be a circuit switch, possibly on the ground.
To address the problems of the interdependence of the terminals and degraded switching agility, justified by increasingly volatile traffic characteristics (unpredictable arrival of data in bursts, short data streams with diversification of destinations), a second solution consists of decoupling the uplink access patterns from the downlink access patterns. The data is grouped into packets which are provided with a header containing an address correlated with a target user terminal. Thanks to this header, the packets are self-switched in the satellite switch. A controller manages access to the uplinks and, after analyzing the address, the switch switches the data on board the satellite and applies statistical multiplexing at each of its outputs. However, statistical management of access to the downlinks leads to the following problem: conflict results if many packets are addressed to the same output at the same time (i.e. must be supported by the same downlink from the satellite to one or more user stations). The conflict is resolved by means of a buffer memory associated with scheduling algorithms. The buffer memory has a finite capacity and if that capacity is exceeded a phenomenon known as congestion results. A first solution to this problem is to increase the size of the buffer memory, representing a penalty in terms of the onboard weight and power consumption balance. A second solution is to include a device for controlling access to the downlinks. The objective of these control mechanisms is to limit the bit rate characteristics of the user terminals whose streams are in competition on a given downlink, either preventively or reactively. This solution is distinguished from the deterministic switching referred to above in the sense that the downlink access pattern is not strictly defined, and the arrangement of the packets remains statistical, although the probability of congestion is reduced by the action of said mechanisms.
From the point of view of onboard complexity, the introduction of the buffer memory and the step of analyzing the destination address have a decisive consequence for onboard implementation. In particular, the use of regenerative processing, which consists of demodulating, decoding (correcting transmission errors), and analyzing the data and then coding and modulating the data using digital technology, is indispensable. These technologies are relatively new, however, and often considered risky. Moreover, the processing capacity of digital equipment leads to a multiplication of the number of equipment units, at the expense of onboard mass and power consumption.
The systems described above are confronted with new demands related to the evolution of the Internet. The increase in the number of autonomous systems, their geographical distribution and the nature of future applications (high bit rates, multiple quality/priority levels, non-connected mode) induce the following constraints:                a requirement for interoperability between networks (reducing adaptation mechanisms),        a high transmission capacity,        hierarchical management of streams in non-connected mode, and        increasingly complex management of addresses (and routes).        
Deterministic switching solutions offer high transmission capacity with relative interoperability because the transmission links are transparent (independent of the waveform). However, management of the streams is somewhat inflexible. Statistical switching solutions offer more flexible management of the streams and addresses, but their processing capacity is low and their waveform dependence represents a penalty in terms of interoperability.
The present invention aims to provide a solution in this direction.
To meet the above requirements, the payload of the satellite must be able to:                switch data at high bit rates (hundreds of megabits per second) from different beams (several tens of beams),        establish and manage dynamically and hierarchically the routes for the packets in transit with no concept of connection (by default, managing the addresses for the switches), and        offer the possibility of managing the protocols of the network layer (i.e. of the Internet) to ensure good integration into the terrestrial networks and relative autonomy (this is known as “seamless” integration).        